Regions of the circulation, such as the carotid bulb, the proximal coronary arteries and the distal aorta are exposed to disturbed, often oscillatory flow and are predisposed to development of atherosclerosis. There is currently no therapy to prevent lesion development at these sites. Tetrahydrobiopterin (BH4) is a critical cofactor for the nitric oxide synthase (NOS) enzymes and in its absence, the NOS enzymes become uncoupled, so that they produce superoxide (O2-) rather than NO. Our laboratory has discovered a new mechanism by which endothelial cells modulate BH4 levels in response to shear. We found that laminar shear stimulates BH4 levels by 30-fold, and increases the activity of GTP cyclohydrolase-1 (GTPCH-1), the rate-limiting enzyme for BH4 production, by a similar extent. Shear stress dissociates GTPCH-1 from its feedback regulatory protein (GFRP), and this allows phosphorylation of GTPCH-1 on serine 81 (serine 72 in the mouse) by casein kinase alpha prime. In contrast, oscillatory shear stress does not dissociate GFRP and GTPCH-1, and does not cause GTPCH-1 phosphorylation, causing BH4 levels to be insufficient. This reduces NO production and increases O2 levels, and markedly enhances atherosclerotic lesion development in a mouse model with disturbed carotid flows. We therefore propose that GTPCH-1 phosphorylation represents a critical switch that alters endothelial cell phenotype, promotes oxidative injury, reduces NO production and predisposes to atherosclerosis. To test this hypothesis in vivo, we have successfully targeted knock-in of both an aspartate (to mimic phosphorylation) and an alanine (to prevent phosphorylation) in murine embryonic stem cells. In aims 1, we will to study mice with the aspartate knock-in (KI[GCH/S72D] mice) to determine if maintaining GTPCH-1 activation prevents atherosclerosis and preserves NO function. In aim 2, we will study mice in which GTPCH-1 cannot be activated by shear ( K I [GCH/S72A] mice), hypothesize that these animals will have enhanced atherosclerosis lesion. In the final aim, we plan to continue studies to discover new molecules that cause dissociation of GFRP and GTPCH-1. To accomplish this, we have developed a high-throughput screening assay and have already used this to screen 34,000 molecules. We have promising preliminary data to show that this approach will allow discovery of genes that can increase endothelial cell BH4 levels. We propose that agents discovered using this approach will provide of novel approach for prevention of atherosclerosis at sites of disturbed flow in vivo.